Proposed Test Rule: id attribute value is unique

Description

This rule checks that all id attribute values on a single page are unique.

Applicability

This rule applies to any id attribute whose value is not an empty string (""), specified on a HTML or SVG element.

Note: Elements that are neither included in the accessibility tree nor visible are still considered for this rule.

Expectation

The value of the attribute is unique across all other id attributes specified on HTML or SVG elements that exist within the same document tree or shadow tree as the element on which the applicable id attribute is specified.

Assumptions

There are currently no assumptions.

Accessibility Support

There are no major accessibility support issues known for this rule.

Background

Test Cases

Passed

Passed Example 1

There is only one id attribute within the document context.

<div id="my-div">This is my first element</div>

Passed Example 2

All id attributes are unique within the document context.

<div id="my-div1">This is my first element</div>
<div id="my-div2">This is my second element</div>
<svg id="my-div3">This is my third element</svg>

Passed Example 3

Two of the id attributes are the same (my-elt), but they are in different trees (the first one in the document tree and the second in the shadow tree).

<div id="my-elt"></div>
<div id="host"></div>
<script>
	var host = document.getElementById('host')
	var shadow = host.attachShadow({ mode: 'open' })
	shadow.innerHTML = '<b id="my-elt"></b>'
</script>

Passed Example 4

Both id attributes are the same (my-elt), but they are in different document trees because the iframe is creating a new one.

<div id="my-elt"></div>
<iframe title="Empty frame" srcdoc="<span id='my-elt'></span>"></iframe>

Failed

Failed Example 1

The id attribute label is not unique among all id attributes in the document, resulting in a wrong programmatic label on the input field.

<div id="label">Name</div>
<div id="label">City</div>

<input aria-labelledby="label" type="text" name="city" />

Failed Example 2

The id attribute label is not unique among all id attributes in the document, resulting in a wrong programmatic label on the input field.

<div id="label">Name</div>
<svg id="label">
	<text x="0" y="15">City</text>
</svg>

<input aria-labelledby="label" type="text" name="city" />

Failed Example 3

The id attribute label is not unique among all id attributes in the document, resulting in a wrong programmatic label on the input field. This rule still considers elements that are neither included in the accessibility tree nor visible.

<span id="label" style="display: none;">Name</span>
<span id="label">City</span>

<input aria-labelledby="label" type="text" name="city" />

Inapplicable

Inapplicable Example 1

There is no id attribute in this document.

<div>This is my first element</div>

Inapplicable Example 2

The xml:id attribute is not considered by this rule.

<div xml:id="my-div">This is my first element</div>

Inapplicable Example 3

These id attributes have an empty value.

<span id="">Hello</span> <span id="">world!</span>

Glossary

Focusable

Elements that can become the target of keyboard input as described in the HTML specification of focusable and can be focused.

Included in the accessibility tree

Elements included in the accessibility tree of platform specific accessibility APIs. Elements in the accessibility tree are exposed to assistive technologies, allowing users to interact with the elements in a way that meet the requirements of the individual user.

The general rules for when elements are included in the accessibility tree are defined in the core accessibility API mappings. For native markup languages, such as HTML and SVG, additional rules for when elements are included in the accessibility tree can be found in the HTML accessibility API mappings (working draft) and the SVG accessibility API mappings (working draft).

For more details, see [examples of included in the accessibility tree][].

Note: Users of assistive technologies might still be able to interact with elements that are not included in the accessibility tree. An example of this is a focusable element with an aria-hidden attribute with a value of true. Such an element could still be interacted using sequential keyboard navigation regardless of the assistive technologies used, even though the element would not be included in the accessibility tree. [examples of included in the accessibility tree]: https://act-rules.github.io/pages/examples/included-in-the-accessibility-tree/

Namespaced Element

An element with a specific namespaceURI value from HTML namespaces. For example an “SVG element” is any element with the “SVG namespace”, which is http://www.w3.org/2000/svg.

Namespaced elements are not limited to elements described in a specification. They also include custom elements. Elements such as a and title have a different namespace depending on where they are used. For example a title in an HTML page usually has the HTML namespace. When used in an svg element, a title element has the SVG namespace instead.

Outcome

An outcome is a conclusion that comes from evaluating an ACT Rule on a test subject or one of its constituent test target. An outcome can be one of the three following types:

Inapplicable: No part of the test subject matches the applicability
Passed: A test target meets all expectations
Failed: A test target does not meet all expectations

Note: A rule has one passed or failed outcome for every test target. When there are no test targets the rule has one inapplicable outcome. This means that each test subject will have one or more outcomes.

Note: Implementations using the EARL10-Schema can express the outcome with the outcome property. In addition to passed, failed and inapplicable, EARL 1.0 also defined an incomplete outcome. While this cannot be the outcome of an ACT Rule when applied in its entirety, it often happens that rules are only partially evaluated. For example, when applicability was automated, but the expectations have to be evaluated manually. Such “interim” results can be expressed with the incomplete outcome.

Programmatic Label

Element L is a programmatic label of target element T if either:

T is a labeled control of L; or
L is referenced by ID in the aria-labelledby attribute of T.

For more details, see examples of programmatic label.

Note: a given element may have more than one programmatic label.

Visible

Content perceivable through sight.

Content is considered visible if making it fully transparent would result in a difference in the pixels rendered for any part of the document that is currently within the viewport or can be brought into the viewport via scrolling.

Content is defined in WCAG.

For more details, see examples of visible.

Implementations

This section is not part of the official rule. It is populated dynamically and not accounted for in the change history or the last modified date.

Implementation	Consistency	Complete	Report
Alfa	Consistent	Yes	View Report
Axe-core	Consistent	Yes	View Report
QualWeb	Consistent	Yes	View Report
SortSite	Consistent	Yes	View Report

Changelog

This is the first version of this ACT rule.